The grinding wheel used on portable grinders generally consists of an abrasive disk having a centrally located bore for receiving an internally threaded collar nut. The collar nut is adapted to be mounted to the externally threaded spindle of the grinder. Typically, a support flange is positioned on the spindle between the grinding wheel and an annular shoulder formed on the spindle to provide backing support for the grinding wheel. The support flange is typically configured to engage the backside of the abrasive disk around its outer radial edge. The direction of rotation of the spindle when the grinder is energized is such that the collar nut will self-thread onto the spindle until a tight frictional engagement is provided between the support flange and the grinding wheel. The grinding wheel can then be further tightened onto, or subsequently removed from, the spindle by applying a wrench to the collar nut.
The collar nut in such conventional assemblies is typically not permanently affixed to the abrasive disk, but rather is intended to be reused when a worn disk is replaced. In addition to the possibility of losing or misplacing the collar nut, this type of assembly is further disadvantageous from the standpoint that replacement abrasive disks must have properly sized bores, which are not uniform for all brands and models. Moreover, the application of driving torque from the spindle to the abrasive disk is solely through the frictional interfaces between the abrasive disk and the spindle directly or between the abrasive disk and the supporting flange and the supporting flange and the spindle. Consequently, under load the abrasive disk subassembly may slip at either of these frictional interfaces. To combat slippage, abrasive disk subassemblies are frequently tightened onto the spindle to such a degree that subsequent removal becomes difficult.
To alleviate some of these problems, various “hubbed”-type abrasive disk subassemblies have been proposed, such as that shown in U.S. Pat. No. 4,694,615 (MacKay, Jr.). Hubbed-type abrasive disk subassemblies include a backing flange that is permanently affixed to the backside of the abrasive disk by attachment to the hub portion of the collar nut using a fastener. The collar nut, backing flange, and fastener become an integral part of the subassembly. The entire subassembly is thus intended to be discarded when the abrasive disk is worn. Hubbed-type grinding wheels are generally intended to be used in combination with specially designed support flanges adapted for engaging driving surfaces on the backing flange affixed to the disk. While the hubbed-type grinding wheels are much less susceptible to slippage problems, they are substantially more expensive than conventional non-hubbed grinding wheels and consequently are not as widely used.
U.S. Pat. No. 5,339,571 (Timmons et al.) discloses an internally threaded collar nut with a shape that is substantially noncircular so as to preclude relative rotation between the abrasive disk and the collar nut. The collar nut is a relatively expensive machined component that in some embodiments is discarded with the worn abrasive disk. The collar nut also includes a head portion that extends above one of the major surfaces of the abrasive disk, potentially interfering with the use of that major surface on a work piece. Due to the mass of abrasive disks, it is believed in the art that a machined collar nut, such as disclosed in the '571 patent, is required.
Accordingly, there is a need for an improved grinding wheel subassembly that provides a positive means of coupling the grinding wheel to the spindle of the grinder without the expense of the hubbed-type wheel subassemblies.